Imaging of corrugated and doped graphene

The graphene is a wonderful material and it can be the realization of two-dimensional electron gas. Indeed when flat geometry is realized it has an electrical conductivity similar to that of copper and unbeaten thermal conductivity. Graphene could pave the way to several technological applications, such as conductive inks, ultrahigh-frequency transistors and photovoltaic devices.  In practice,  it deviates from planar geometry because of ripples probably induced also by undesired impurities (*). Few days ago, scientists from the University at Buffalo, the National Institute of Standards and Technology (NIST), the Molecular Foundry at Lawrence Berkeley National Laboratory (Berkeley Lab), and SEMATECH, published on Nature C0mmunications the results of a research that with X-ray and synchrotron radiation techniqes allowed the imaging of  the local electronic corrugations alongside inhomogeneously doped regions of single graphene sheet. Here the press release from UB.

Beside scanning transmission X-ray microscopy (STXM) imaging, and in situ near edge X-ray absorption fine structure (NEXAFS) spectroscopy experiments, the combined use of density functional theory calculations helped to improve the understanding of deviations from the ideal behavior of graphene spectra.

(*) Update: in general ripples are formed on graphene due to random corrugations that compensates random thermal fluctuations and because of the substrate. However in this experiment samples are doped.

Update 2: A recent viewpoint by Michael Baum confirmed the vision that contaminants and ripples (or wrinkles) are connected. The electrical properties of graphene are so degraded by contaminants, that are believed to be just physisorbed rather than chemisorbed (like in doping). Once understood this we have also physical processes able to free samples by contaminants and restore the conductivity of pure graphene, e.g. by heating.

In conclusion the ideal behavior of graphene is corrupted by contaminants, and new ways to eliminate them are required  if we want to exploit the wonderful properties of this innovative material.

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About Francesco Buonocore

Francesco Buonocore is an Italian solid state theoretical physicist, staff scientist of the Italian government agency ENEA. Francesco likes Physics, thinking about Physics and how Physics and life are related.
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5 Responses to Imaging of corrugated and doped graphene

  1. amanatal says:

    I could not understand this statement “deviates from planar geometry because of ripples probably induced by undesired impurities”. What does impurities have to do with atomic structure ?

  2. First, anamatal thank you to be the first to comment on my blog. Next, my answer: in the abstract on Nature Communications is stated that “local electronic corrugations, visualized as distortions of the π*cloud, have been imaged alongside inhomogeneously doped regions”. In my experience doping and vacancies can induce deformations over the surface of carbon nanotubes because they break sp2 hybridization (for graphene could be the same), so I have correlated doped regions to corrugations. However I don’t know if the authors of the paper have my same vision. Unfortunately at the moment I cannot get the full paper. I suppose also the substrate can play an important role.

  3. amanatal says:

    Got it. I forget that doping create vacancies. It is a concept I read in semiconductors .

  4. Of course doping doesn’t create vacancies every time. Just it can, and in general doping can induce morphological changes. However in my comment before I have neglected random corrugations that compensates random fluctuations. In fact following Landau and Lifschitz’s book of statistical phisics, 2D electron gas could not exist because thermal fluctuations should destroy long-range order, but corrugations serve to stabilize them. I have updated my post to explicit this. In conclusion in my opinion doping could have introduced ripples in the sample that are of distinct nature with respect to those induced by substrate and thermal fluctuations. Maybe UPS and NEXAFS measures of ripples are available for not doped samples. Does someone know?

  5. I have added an update where ripples and degraded electrical performances in graphene are correlated to contaminants.

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